The Development of a Rectal Enema As Microbicide (DREAM) Program addresses the critical need to develop a highly effective, safe, and acceptable microbicide enema with the promise of greater adherence as a more behaviorally-transparent alternative for the prevention of rectal HIV infection. The overall goal of the program is to develop a single dose rectal enema to deliver a tenofovir (TFV) prodrug capable of providing one week of protection. This strategy builds upon proven high levels of efficacy of TFV-based pre-exposure prophylaxis (PrEP) in adherent persons and directly targets the greatest weakness of PrEP regimens - prophylactic failure due to poor adherence. Given the common practice of rectal douching with an enema prior to receptive anal sex, we have selected a dosing strategy which requires little or no behavioral change compared to other oral and topical approaches. Topical delivery also significantly reduces systemic exposure compared to oral and injectable approaches. We propose pharmacokinetic enhancements to increase TFV bioavailability and provide sustained release. Three TFV prodrugs with far greater tissue and cellular uptake than TFV itself - TFV disoproxil fumarate, TFV alafenamide fumarate (TAF, formerly GS7340), and CMX157 - have been selected for study. Integrated with 3 other projects and 3 cores in the DREAM Program, Project 1 involves a series of clinical studies to achieve the following specific aims: Aim 1: Determine the combination of TFV dose and enema tonicity best able to achieve our colon tissue target drug concentrations. Aim 2: Compare TFV enema dosing before and after to recommend dose timing and the impact of semen on tissue drug concentrations. Aim 3: Evaluate the use of biomarkers for both enema dosing and sexual activity to provide objective evidence of adherence to protocol prescribed dosing. Aim 4: Provide clinical samples (plasma, PBMC, colon tissue/cells, rectal fluid) to support method development and assay validation in Project 2, Project 3, and Core C.